Phase Noise Measurement in a Cascaded Radar System

ABSTRACT

A cascaded radar system is provided that includes a master radar system-on-a-chip (SOC) with transmission signal generation circuitry and a slave radar SOC coupled to an output of the master radar SOC to receive a signal from the transmission signal generation circuitry of the master SOC. In this system, the slave radar SOC is operable to measure phase noise in the signal received from the transmission signal generation circuitry of the master SOC.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

Embodiments of the present disclosure generally relate to radar systems,and more specifically relate to measurement of phase noise in a cascadedradar system.

2. Description of the Related Art

A new class of safety systems, referred to as advanced driver assistancesystems (ADAS), has been introduced into automobiles to reduce humanoperation error. These systems are enabled by smart sensors basedprimarily on millimeter-wave automotive radars. The proliferation ofsuch assistance systems, which may provide functionality such asrear-view facing cameras, electronic stability control, and vision-basedpedestrian detection systems, has been enabled in part by improvementsin microcontroller and sensor technologies. Enhanced embeddedradar-based solutions are enabling complementary safety features forADAS designers.

In an automotive radar system, one or more radar sensors may be used todetect obstacles around the vehicle and the speeds of the detectedobjects relative to the vehicle. A processing unit in the radar systemmay determine the appropriate action needed, e.g., to avoid a collisionor to reduce collateral damage, based on signals generated by the radarsensors. Current automotive radar systems are capable of detectingobjects and obstacles around a vehicle, the position of any detectedobjects and obstacles relative to the vehicle, and the speed of anydetected objects and obstacles relative to the vehicle. Via theprocessing unit, the radar system may, for example, alert the vehicledriver about potential danger, prevent a collision by controlling thevehicle in a dangerous situation, take over partial control of thevehicle, or assist the driver with parking the vehicle.

Automotive radar systems are required to meet the functional safetyspecifications of International Standard 26262 titled “RoadVehicles—Functional Safety.” ISO 26262 defines functional safety as theabsence of unreasonable risk caused by malfunctioning behavior ofelectrical/electronic systems. Functional safety in automotive radar isthe prevention of harm to humans due to failure of components in theradar. For automotive radar, the radar should be known to be functioningappropriately within a fault tolerant time interval of approximately 100milliseconds (ms). Thus, while the vehicle is operating, a failure inany part of the radar that would lead to a degraded signal-to-noiseratio (SNR) should be detected, and an appropriate response performedwithin approximately 100 ms.

Phase noise has an important degradation effect in automotive radarsystems such as Frequency Modulated Continuous Wave (FMCW) radar systemsas an unacceptable level of phase noise may mask out objects with weakersignals since objects are detected at frequency offsets of the carrier.Automotive radar systems are typically designed to minimize phase noisebut when in use in a vehicle, the phase noise may degrade (increase) dueto a failure in the system, e.g., a transistor or piece of metal breaks.Therefore, to meet functional safety standards, it is important tomonitor phase noise during vehicle operation and generate an appropriateresponse when the phase noise degrades to an unacceptable level. Atypical requirement is to alert the vehicle driver if phase noiseincreases by approximately 12 decibels (dB).

SUMMARY

Embodiments of the present disclosure relate to methods and apparatusfor phase noise measurement in a cascaded radar system. In one aspect, acascaded radar system is provided that includes a master radarsystem-on-a-chip (SOC) including transmission signal generationcircuitry, and a slave radar SOC coupled to an output of the masterradar SOC to receive a signal from the transmission signal generationcircuitry, the slave radar SOC operable to measure phase noise in thesignal.

In one aspect, a method for phase noise measurement in a cascaded radarsystem is provided that includes receiving a signal in a first radardevice of the cascaded radar system, the signal generated by firsttransmission signal generation circuitry of a second radar device, andmeasuring phase noise in the signal in the first radar device.

In one aspect, a radar device configured to operate in a normal mode anda phase noise measurement mode is provided that includes firsttransmission signal generation circuitry operable to generate areference signal when the radar device is in phase noise measurementmode and to generate signals for transmission when the radar device isin normal mode, an input buffer coupled to second transmission signalgeneration circuitry of another radar device to receive a signal forphase noise measurement from the second transmission signal generationcircuitry when the radar device is in phase noise measurement mode, atleast one receive channel including a baseband filter chain, and phasedetection circuitry coupled to an output of the first transmissionsignal generation circuitry to receive the reference signal, to anoutput of the input buffer to receive the signal from the input buffer,and to an input of the baseband filter chain, the phase detectioncircuitry operable when the radar device is in phase noise measurementmode to generate a phase noise test signal of a phase difference betweenthe signal and the reference signal and to send the phase noise testsignal to the input of the baseband filter chain.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments will now be described, by way of example only,and with reference to the accompanying drawings:

FIG. 1, FIG. 2, and FIG. 3 are block diagrams illustrating an examplecascaded Frequency Modulated Continuous Wave (FMCW) radar system; and

FIG. 4 is a flow diagram of a method for phase noise measurement in acascaded radar system.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

Specific embodiments of the disclosure will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency.

As previously mentioned, degradation (increase) in phase noise in anautomotive radar system such as a FMCW radar system is a functionalsafety risk as an increase in phase noise can significantly degrade thedynamic range of the radar, allowing objects with stronger returns tomask objects with weaker returns. Embodiments of the disclosure providefor high accuracy phase noise measurement in a cascaded radar system asthe radar system is used in an operating vehicle. More specifically,phase noise measurement is provided for a cascaded radar systemimplemented with a master radar system-on-a-chip (SOC) and one or moreslave radar SOCs. In such a system, transmission signal generationcircuitry such as, for example, a radio frequency synthesizer, in themaster radar SOC provides the transmission signal (which may alsoinclude the local oscillator (LO) signal) for the master radar SOC andthe one or more slave radar SOCs. As is explained in more detail herein,at least one of the slave radar SOCs is configured to measure the phasenoise in the transmission signal generation circuitry of the master SOC.In some embodiments, the master radar SOC may also be configured tomeasure the phase noise in the transmission signal generation circuitryof the slave SOC.

FIGS. 1, 2, and 3 are block diagrams of an example cascaded FMCW radarsystem 100 configured to perform phase noise measurement duringoperation of the radar system 100 in a vehicle. FIG. 1 illustrates thetop level architecture of the cascaded radar system 100, FIG. 2illustrates an example FMCW radar SOC suitable for use as the masterradar SOC 102 and slave radar SOC 104 of FIG. 1, and FIG. 3 provides amore detailed view of the configuration of the radar system 100 forphase noise measurement.

Referring now to FIG. 1, the example cascaded FMCW radar system 100includes a master radar SOC 102, a slave radar SOC 104, a processingunit 106, and a network interface 108. The master radar SOC 102 and theslave radar SOC 104 each have the architecture of the example FMCW radarSOC of FIG. 2. Further, the master radar SOC 102 is coupled to the slaveradar SOC 104 to synchronize the operation of the slave radar SOC 104with that of the master radar SOC 102. The master radar SOC 102 and theslave radar SOC 104 are referred to collectively herein as the radarsystem frontend or the frontend.

The master radar SOC 102 and the slave radar SOC 104 are coupled to theprocessing unit 106 via a high speed serial interface. As is explainedin more detail in reference to FIG. 2, each radar SOC 102,104 includesfunctionality to generate multiple digital beat signals (alternativelyreferred to as dechirped signals, intermediate frequency (IF) signals,or raw radar signals) that are provided to the processing unit 106 viathe high speed serial interface.

The processing unit 106 includes functionality to process the receivedbeat signals to determine, for example, distance, velocity, and angle ofany detected objects. The processing unit 106 may also includefunctionality to perform post processing of the information about thedetected objects, such as tracking objects, determining rate anddirection of movement, etc. The processing unit 106 may include anysuitable processor or combination of processors as needed for theprocessing throughput of the application using the radar data. Forexample, the processing unit 106 may include a digital signal processor(DSP), a microcontroller (MCU), an SOC combining both DSP and MCUprocessing, or a field programmable gate array (FPGA) and a DSP.

The processing unit 106 provides control information as needed to one ormore electronic control units in the vehicle via the network interface108. Electronic control unit (ECU) is a generic term for any embeddedsystem in a vehicle that controls one or more the electrical system orsubsystems in the vehicle. Types of ECU include, for example,electronic/engine control module (ECM), powertrain control module (PCM),transmission control module (TCM), brake control module (BCM or EBCM),central control module (CCM), central timing module (CTM), generalelectronic module (GEM), body control module (BCM), and suspensioncontrol module (SCM).

The network interface 108 may implement any suitable protocol, such as,for example, the controller area network (CAN) protocol, the FlexRayprotocol, or Ethernet protocol.

Referring now to FIG. 2, the example FMCW radar SOC 200 depicted isconfigured to be used as the master radar SOC 102 or the slave radar SOC104 in the radar system 100. Further, the radar SOC 200 is configured toperform phase noise measurement of the radio frequency synthesizer of amaster radar SOC when used as a slave radar SOC.

The radar SOC 200 may include multiple transmit channels 204 fortransmitting FMCW signals and multiple receive channels 202 forreceiving the reflected transmitted signals. Further, the number ofreceive channels may be larger than the number of transmit channels. Forexample, an embodiment of the radar SOC 200 may have two transmitchannels and four receive channels. A transmit channel includes asuitable transmitter and antenna. A receive channel includes a suitablereceiver and antenna. Further, each of the receive channels 202 areidentical and include a mixer 206, 208 to mix the transmitted signalwith the received signal to generate a beat signal, a baseband bandpassfilter 210, 212 for filtering the beat signal, a variable gain amplifier(VGA) 214, 216 for amplifying the filtered beat signal, and ananalog-to-digital converter (ADC) 218, 220 for converting the analogbeat signal to a digital beat signal. The bandpass filter, VGA, and ADCof a receive channel may be collectively referred to as a baseband chainor baseband filter chain.

The receive channels 202 are coupled to a digital front end (DFE) 222that performs decimation filtering on the digital beat signals to reducethe data transfer rate. The DFE 222 may also perform other operations onthe digital beat signals, e.g., DC offset removal. The DFE 222 iscoupled to a high speed serial interface (I/F) 224 that transfers theoutput of the DFE 222 to the processing unit 106.

The control module 226 includes functionality to control the operationof the radar SOC 200 in normal mode and in phase noise measurement mode.The control module 226 may include, for example, a buffer to store theoutput samples of the DFE 222, an FFT (Fast Fourier Transform) engine tocompute spectral information of the buffer contents, and an MCU thatexecutes firmware to control the operation of the radar SOC 200 innormal mode and in phase noise measurement mode. Functionality of thecontrol module 226 is described in more detail in reference to FIGS. 3and 4.

The serial peripheral interface (SPI) 228 provides an interface forcommunication with the processing unit 106. For example, the processingunit 106 may use the SPI 228 to send control information, e.g., timingand frequencies of chirps, output power level, triggering of monitoringfunctions such as phase noise monitoring, etc., to the radar SOC 200.The radar SOC 200 may use the SPI 228, for example, to send the resultsof phase noise measurement and other monitoring functions to theprocessing unit 106.

The radio frequency synthesizer (RFSYNTH) 230 includes functionality togenerate FMCW signals for transmission. In some embodiments, the RFSYNTH230 includes a phase locked loop (PLL) with a voltage controlledoscillator (VCO), and a programmable ramp generator that is programmedby the control module 226. If the radar SOC 230 is used as the slaveradar SOC, the RFSYNTH 230 is active when the radar system frontend isin phase noise measurement mode and is not active when the radar systemfrontend is operating in normal mode.

The multiplexer 232 is coupled to the RFSYNTH 230 and the input buffer236. If the radar SOC 200 is used as the slave radar SOC 104, the radarSOC 200 receives signals generated by the RFSYNTH of the master radarSOC 102 via the buffer 236. The multiplexer 232 is configurable toselect between signals received in the input buffer 236 and signalsgenerated by the RFSYNTH 230.

The output buffer 238 is coupled to the multiplexer 232 and may be usedto transmit signals selected by the multiplexer 232 to the input bufferof another radar SOC.

The clock multiplier 240 increases the frequency of the transmissionsignal (LO signal) to the LO frequency of the mixers 206, 208. In theradar system 100, the transmission signal for the master radar SOC 102and the slave radar SOC 104 is generated by the RFSYNTH of the masterradar SOC 102.

The clean-up PLL (phase locked loop) 234 operates to increase thefrequency of the signal of an external low frequency reference clock(not shown) to the frequency of the RFSYNTH 234 and to filter thereference clock phase noise out of the clock signal.

The test mixer 242, e.g., a linear (real) mixer, receives input signalsfrom the RFSYNTH 230 via the multiplexer 232 and the input buffer 236.The output of the text mixer 242 is coupled to the input of the bandpassfilter 212. The output signal of the test mixer 242, which may bereferred to as the phase noise test signal herein, is the differencefrequency between the two input signals and the sum of the phase noisein the two input signals. The test mixer is activated on the radar SOC200 when the radar SOC is the slave radar SOC 104 and the radar systemfrontend is in phase noise measurement mode. In some embodiments, thetest mixer 242 is not used when the radar SOC 200 is the master radarSOC 102.

Referring now to FIG. 3, the block diagram provides a more detailed viewof the configuration of the radar system 100 for measuring phase noisein the RFSYNTH 302 of the master radar SOC 102. An external clock (notshown) provides a reference clock signal for both the master RFSYNTH 302and the slave RFSYNTH 314 via the master clean-up PLL 304 and the slaveclean-up PLL 316. The output buffer 308 of the master radar SOC 102 iscoupled to the input buffer 320 of the slave radar SOC 104. In someembodiments, the output buffer 320 of the slave radar SOC 104 and theinput buffer 306 of the master radar SOC 102 are not coupled externallyand are not used.

The master multiplexer 312 is configured to select the output signal ofthe master RFSYNTH 302 in both normal mode and phase noise measurementmode. This output signal is sent to the slave radar SOC 104 via themaster output buffer 308. When the radar system frontend is in normalmode, the master RFSYNTH 302 is programmed by the master control module(not shown) to generate FMCW signals for transmission. The generatedsignals are sent to the slave radar SOC 104 via the output buffer 308and are provided to the master transmit and receive channels (not shown)via the clock multiplier 310. Configuration of the master RFSYNTH 302when the radar system frontend is in phase noise measurement mode isdescribed in reference to the method of FIG. 4.

When the radar system frontend is in normal mode, the slave RFSYNTH 314is not active and the slave multiplexer 324 is configured to select theoutput of the slave input buffer 320. Further, the test mixer 326 is notactive. The FMCW signals from the master radar SOC 102 are provided tothe slave transmit and receive channels (not shown) via the clockmultiplier 322. Configuration of the slave RFSYNTH 314 and the slavemultiplexer 324 when the radar system frontend is in phase noisemeasurement mode is described in reference to the method of FIG. 4.

FIG. 4 is a flow diagram of a method for phase noise measurement in acascaded radar system such as the radar system of FIGS. 1, 2, and 3. Themethod is described in reference to the block diagrams of these figures.The method can be performed while the radar system is in use in anoperating vehicle. For example, the method may be performed during thetime period the processing unit 100 is processing a frame of radar datareceived from the frontend.

To perform the phase noise measurement, the frontend of the radar system100 is activated in phase noise measurement mode. Once this mode isactivated, the control module of the slave radar SOC 104 configures 400the SOC for phase noise measurement. This configuration process includesdisabling the slave receiver mixer 208 of the receiver channel to beused for processing the phase noise test signal during the phase noisemeasurement, enabling the slave test mixer 326, and coupling the outputof the slave mixer 326 to the input of the bandpass filter 212 in thereceiver channel to be used for processing the phase noise test signaloutput by the slave test mixer 326. This configuration process furtherincludes coupling the slave multiplexer 324 to the output of the slaveRFSYNTH 314 rather than to the slave input buffer 320.

Further, the control module of the master radar SOC 102 configures 402the master RFSYNTH 302 to output a signal at a constant frequencyf_(LO), e.g., 19 GHz (gigaHertz). This signal may be referred to as thesignal-under-test (SUT) herein. The SUT is one input to the slave testmixer 326.

The slave control module further configures the slave SOC to measure theamplitude A of the phase noise test signal output by the slave testmixer 206. More specifically, the slave control module configures 404the slave RFSYNTH 314 to output a reference signal at frequencyf_(LO)+f_(IF1), where f_(IF1) is in the passband of the bandpass filter.The intermediate frequency f_(IF1) may be, for example, 1 MHz(megaHertz). The slave control module further sets 406 the gain of theVGA in the receiver channel to a gain G₁, e.g., 0 dB. With thisconfiguration, the slave control module measures 408 the amplitude A ofthe phase noise test signal at f_(IF1). Note that because the output ofthe slave test mixer 326 is coupled to the input of the bandpass filterof a receiver channel, the phase noise test signal is processed in asimilar fashion to a signal received by one of the receivers, i.e., thesignal is filtered, amplified, and converted from an analog signal to adigital signal prior to measuring the amplitude.

The slave control module then configures the slave SOC 104 to measurethe total noise power N in the phase noise test signal. Morespecifically, the slave control module configures 410 the slave RFSYNTH314 to output a reference signal at frequency t_(LO)+f_(IF2), wheref_(IF2) is in the lower stopband of the bandpass filter. In general, abandpass filter with a center frequency f_(c) has two stopbands with0<stopband 1<f_(c)<stopband 2. The lower stopband is stopband 1. Theintermediate frequency f_(IF2) may be, for example, 10 kHz (kilohertz).The slave control module further sets the gain of the VGA in thereceiver channel to a gain G₂, e.g., 26 dB. In general, G₂>G₁. With thisconfiguration, the slave control module measures 414 the total noisepower N over [f_(IG1)−Δ, f_(IF1)+α] where the value of Δ is selected asa tradeoff between computation time (smaller Δ takes longer to measure)and accuracy (larger Δ is less accurate). In some embodiments, Δ=10 kHz.

For the amplitude measurement, the value of f_(IF1) is chosen to be nearthe frequency offset where phase noise degradation is problematic. Thisis typically in the passband of the bandpass filter. For the measurementof the total noise power, the value of f_(IF2) is chosen to be somewherein the stop band of the bandpass filter. Thus, during the total noisepower measurement, the amplitude of the carrier (at f_(IF2)) issuppressed by the bandpass filter, but the amplitude of the phase noise(near f_(IF1)) is allowed to pass through. This significantly relaxesthe dynamic range of the ADC because the ADC does not have tosimultaneously handle the full scale carrier and phase noise in a singlemeasurement.

During the amplitude measurement, the carrier at f_(IF1) is very largeand may saturate the ADC. Therefore, the gain G₁ is chosen to be smallso that the ADC does not saturate while the carrier is in the bandpassfilter passband. During the total noise power measurement, the carrieris in the stopband of the filter and heavily attenuated before the ADC.Therefore the VGA gain can be increased to G2>G1 so that the phase noiseis amplified. The value of G2 is chosen so that the phase noise isamplified beyond the noise floor of the ADC. The expectation is that theratio G2/G1 is known.

The slave control module then computes 416 the phase noise PN based onthe measured total noise power N, the amplitude A, and the two gainvalues, i.e.,

PN=N−10log10(Δ)−A−G ₂ +G ₁,

assuming N and A are in dBm (Decibel milliwatts), and G1 and G2 are indB. The computed phase noise measurement may then be used to determinewhether the current level of phase noise in the master RFSYNTH 302 isacceptable. For example, the value of PN may be compared to a noisethreshold to decide whether or not the current amount of phase noise isacceptable.

Once the phase noise measurement is complete, the slave control modulereconfigures the slave SOC 104 for normal operation. Suchreconfiguration includes coupling the slave multiplexer 324 to the slaveinput buffer 320, disabling the slave RFSYNTH 314, enabling the slavereceiver mixer that was disabled, and disconnecting the output of theslave mixer 326 from the input of the bandpass filter.

Other Embodiments

While the disclosure has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the disclosure as disclosed herein.

For example, embodiments have been described herein in which the phasenoise measurement processing is performed in the control module of theslave radar SOC. One of ordinary skill in the art will understandembodiments in which some or all of the phase noise measurementprocessing is performed external to the SOC, e.g., by the processingunit or by an external MCU.

In another example, embodiments have been described herein in which aclock multiplier is used. One of ordinary skill in the art willunderstand embodiments in which the multiplier is not needed because theRFSYNTH operates at the LO frequency rather than a lower frequency.

In another example, embodiments have been described herein in which aninternal FFT engine in the slave control module is used in thecomputation of the amplitude of the phase noise test signal. One ofordinary skill in the art will understand embodiments in which theamplitude is measured by a received signal strength indicator (RSSI) inthe DDE.

In another example, embodiments have been described herein in which thetransmission signal generation circuitry is assumed to a radio frequencysynthesizer. One of ordinary skill in the art will understandembodiments in which this circuitry is an open loop oscillator (radiofrequency oscillator) plus a digital-to-analog converter (DAC) or othersuitable transmission signal generation circuitry.

In another example, embodiments have been described herein in which areal mixer is used for phase difference detection. One of ordinary skillin the art will understand embodiments in which the real mixer isreplaced with a complex mixer or any suitable circuitry that can detectphase difference. In embodiments using a complex mixer, the value off_(IF2) can be set to 0.

In another example, embodiments have been described herein in which thecascaded radar system has a single slave radar SOC. One of ordinaryskill in the art will understand embodiments in which the cascaded radarsystem includes multiple slave radar SOCs.

In another example, embodiments have been described herein in which theslave SOC measures phase noise in the master SOC. One of ordinary skillin the art will understand embodiments in which the roles of the slaveSOC and the master SOC are reversed to test for a latent fault in theslave SOC, i.e., a failure in the phase noise measurement path. Forexample, if the mixer 242 in the slave SOC is not working, the radarsystem would still work, but the ability to catch a subsequent failureof the master RFSYNTH would be compromised. Latent faults are lessimportant than single point faults (the failure of the RFSYNTH is thelatter), but such faults are still relevant. Latent fault coverage ofthe slave test mixer 326 and the slave RFSYNTH 314 can be provided byreversing the entire process. Specifically, referring to FIG. 3, theoutput of the slave output buffer 318 may be coupled to the input of themaster input buffer 306 and the roles of the master SOC and the slaveSOC in the overall phase noise measurement may be reversed.

Certain terms are used throughout the description and the claims torefer to particular system components. As one skilled in the art willappreciate, components in radar systems may be referred to by differentnames and/or may be combined in ways not shown herein without departingfrom the described functionality. This document does not intend todistinguish between components that differ in name but not function. Inthe following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .” Also, theterm “couple” and derivatives thereof are intended to mean an indirect,direct, optical, and/or wireless electrical connection. Thus, if a firstdevice couples to a second device, that connection may be through adirect electrical connection, through an indirect electrical connectionvia other devices and connections, through an optical electricalconnection, and/or through a wireless electrical connection, forexample.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope ofthe disclosure.

What is claimed is:
 1. A cascaded radar system comprising: a masterradar system-on-a-chip (SOC) including first transmission signalgeneration circuitry; and a slave radar SOC coupled to an output of themaster radar SOC to receive a first signal from the first transmissionsignal generation circuitry, the slave radar SOC operable to measurephase noise in the first signal.
 2. The cascaded radar system of claim1, in which the slave radar SOC includes second transmission signalgeneration circuitry operable to generate a first reference signal formeasuring phase noise in the first signal, and first phase detectioncircuitry coupled to the output of the master radar SOC and to an outputof the second transmission signal generation circuitry to receive thefirst reference signal, the first phase detection circuitry operable togenerate a first phase noise test signal of a phase difference betweenthe first signal and the first reference signal.
 3. The cascaded radarsystem of claim 2, in which the first phase detection circuitry is alinear mixer.
 4. The cascaded radar system of claim 2, in which anoutput of the first phase detection circuitry is coupled to an input ofa bandpass filter in a receiver channel of the slave radar SOC, thebandpass filter operable to filter the first phase noise test signalprior to measurement of the phase noise in the first phase noise testsignal.
 5. The cascaded radar system of claim 1, in which the masterradar SOC is coupled to an output of the slave radar SOC to receive asecond signal from the second transmission signal generation circuitry,the master radar SOC operable to measure phase noise in the secondsignal.
 6. The cascaded radar system of claim 5, in which the firsttransmission signal generation circuitry is operable to generate asecond reference signal for measuring phase noise in the second signal,and the master radar SOC includes second phase detection circuitrycoupled to the output of the slave radar SOC and to an output of thefirst transmission signal generation circuitry to receive the secondreference signal, the second phase detection circuitry operable togenerate a second phase noise test signal of a phase difference betweenthe second signal and the second reference signal.
 7. The cascaded radarsystem of claim 5, in which the second phase detection circuitry is alinear mixer.
 8. The cascaded radar system of claim 5, in which anoutput of the second phase detection circuitry is coupled to an input ofa second bandpass filter in a receiver channel of the master radar SOC,the second bandpass filter operable to filter the second phase noisetest signal.
 9. The cascaded radar system of claim 1, in which the firsttransmission signal generation circuitry and the second transmissionsignal generation circuitry each include a radio frequency synthesizer.10. The cascaded radar system of claim 1, in which the master radar SOCand the slave radar SOC are identical SOCs.
 11. A method for phase noisemeasurement in a cascaded radar system, the method comprising: receivinga signal in a first radar device of the cascaded radar system, thesignal generated by first transmission signal generation circuitry of asecond radar device; and measuring phase noise in the signal in thefirst radar device.
 12. The method of claim 11, in which measuring phasenoise comprises: generating a reference signal by second transmissionsignal generation circuitry in the first radar device; generating aphase noise test signal as a phase difference of the signal and thereference signal by first phase detection circuitry in the first radardevice; and measuring the phase noise in the signal based on the phasenoise test signal.
 13. The method of claim 12, in which measuring thephase noise in the signal based on the phase noise test signalcomprises: measuring an amplitude of the phase noise test signal;measuring total noise power of the phase noise test signal; andcomputing the phase noise based on the amplitude and the total noisepower.
 14. The method of claim 11, in which the first radar device andthe second radar device are identical radar systems-on-a-chip.
 15. Themethod of claim 11, in which the first radar device is a slave radarsystem-on-a-chip (SOC) in a cascaded radar system and the second radardevice is a master radar SOC in the cascaded radar system.
 16. Themethod of claim 11, in which the first radar device is a master radarSOC in a cascaded radar system and the second radar device is a slaveradar SOC in the cascaded radar system.
 17. A radar device configured tooperate in a normal mode and a phase noise measurement mode, the radardevice comprising: first transmission signal generation circuitryoperable to generate a reference signal when the radar device is inphase noise measurement mode and to generate signals for transmissionwhen the radar device is in normal mode; an input buffer coupled tosecond transmission signal generation circuitry of another radar deviceto receive a signal for phase noise measurement from the secondtransmission signal generation circuitry when the radar device is inphase noise measurement mode; at least one receive channel including abaseband filter chain; and phase detection circuitry coupled to anoutput of the first transmission signal generation circuitry to receivethe reference signal, to an output of the input buffer to receive thesignal from the input buffer, and to an input of the baseband filterchain, the phase detection circuitry operable when the radar device isin phase noise measurement mode to generate a phase noise test signal ofa phase difference between the signal and the reference signal and tosend the phase noise test signal to the input of the baseband filterchain.
 18. The radar device of claim 17, in which the radar device andthe another radar device are identical radar systems-on-a-chip.
 19. Theradar device of claim 17, in which the first transmission signalgeneration circuitry and the second transmission signal generationcircuitry each include a radio frequency synthesizer.
 20. The radardevice of claim 17, in which the phase detection circuitry is a linearmixer.